1. Field of the Invention
The present invention relates to a curable composition, an image forming material, and a planographic printing plate precursor.
2. Description of the Related Art
Photopolymerizable compositions in which a photopolymerization initiator is mixed with a polymerizable compound having an addition polymerizable unsaturated group are photopolymerized by irradiation with active rays such as ultraviolet rays. Photopolymerizable compositions have been used as photosensitive materials for forming images by utilizing changes in the physical properties thereof concomitant with polymerization, such as changes in solubility or adhesion.
When using photosensitive materials including such polymerizable compositions to form an image, it is known that radical polymerization reactions in the photopolymerizable compositions are inhibited in the presence of oxygen, causing a drop in the sensitivity of the materials, fluctuations in sensitivity due to fluctuations in oxygen concentration, and various other problems. To address these problems, a transparent film called a cover sheet is adhered to the surface of the photosensitive material, or a polymer layer called an overcoat is applied to the photosensitive material, to prevent oxygen from contacting the photosensitive layer.
However, the method of adhering a cover sheet onto the surface of a photosensitive material to prevent oxygen from contacting with the photosensitive layer has not been considered preferable since it is necessary to include an extra step of adhering the cover sheet to the photosensitive layer surface upon producing the photosensitive material; and, when the resultant photosensitive material is used, light is scattered at the interface between the cover sheet and the photosensitive layer when the material is exposed to light, leading to deterioration in image quality; thus, the cover sheet should be peeled off and removed before development, leading to increased costs and labor.
In order to apply an overcoat layer to the surface of a photosensitive material to prevent oxygen from contacting the photosensitive layer, a method has been used wherein a transparent polymer that has a low oxygen permeability and is soluble in a developer is dissolved in a solvent in which the photosensitive layer is insoluble; this solution is subsequently applied to the photosensitive layer and dried. As the polymer, polyvinyl alcohol (PVA) has generally been used. However, this method also has problems in that equipment for attaining multi-coating is required when the photosensitive material is produced; and, at the time of the coating, a coating solution for the overcoat layer penetrates into the photosensitive layer so that the performance of the photosensitive layer is deteriorated, and the like.
As a method for suppressing the effect of oxygen on a photosensitive layer, a method of incorporating a specific binder into the photosensitive layer has been known (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 8-160612). A method of incorporating a specific amine compound as a polymerization promoter into a photosensitive layer to improve the sensitivity of the photosensitive layer is also known (see, for example, JP-A No. 7-165808).
However, it has been difficult to suppress the polymerization inhibiting effect of the oxygen by using the specific binder described in JP-A No. 8-160612, or the amine compound described in JP-A No. 7-165808.